When light strikes an object, visual feature information is transferred to light scattered from the object. The scattered light comprises various wave vectors of propagating and evanescent wave components. The propagating waves carrier large feature information that reaches the far field and can be collected using lenses and an image sensor to reconstruct an image of the object. By contrast, evanescent waves carry more detailed visual information regarding relatively smaller features of the object, but evanescent waves exponentially decay and are confined to the near field of the object. Thus, when a conventional lens is used to collect the light scattered from an object, the evanescent waves are lost before reaching the image plane. This inability to capture evanescent waves scattered from features smaller than half the illumination wavelength is called the “diffraction limit.”
FIG. 1 shows a schematic representation of the operation of a conventional lens 102. Curve 104 represents propagating waves scattered from an object plane of the object. The propagating waves carry large feature information, pass through the lens 102 and reach the image plane in the far field, where the large feature information of the object are reproduced in an image of the object that can be observed or collected in the far field. On the other hand, curve 106 represents exponential decay of the evanescent waves scattered from the object plane in the near field. Any fine feature visual information contained in the evanescent waves is substantially lost before reaching the lens 102.
In recent years, superlenses have been proposed with the potential to recover lost evanescent waves. This is accomplished by coupling evanescent waves scattered from the object to surface excitations on a slab of negative refractive index material. A superlens compensates for evanescent wave decay in the near field of the object using the strong enhancement provided by the surface excitations, which restore the evanescent wave components. However, typical superlenses are capable of projecting small feature information into the near field of the superlens.
FIG. 2 shows a schematic representation of the operation of a metamaterial superlens 202. Propagating waves pass through the superlens 202 into the far field, as described above for the conventional lens 102. However, unlike the conventional lens 102, evanescent waves are magnified within the superlens 202, as indicated by exponentially increasing curve 204. FIG. 2 includes a dashed line that conceptually separates the near field of the superlens 202 from the far field. The superlens 202 projects the evanescent waves magnified within the superlens 202 into the near field of the superlens 202 where the evanescent waves exponentially decay again, as indicated by curve 206.
One disadvantage of metamaterial superlenses is that they typically are capable of projecting small feature information into the near field of the superlens. Thus, fine feature information of the object can only be observed in the near field of the superlens and cannot be practically observed in the far field. A second disadvantage of superlenses is negative permeability u can be difficult to achieve for radiation in the visible portion of the electromagnetic spectrum. Thus, in practice, a slab of plasmonic material with a negative permittivity 8 can be used as a superlens for radiation in the visible spectrum. Such plasmonic-based superlenses are only capable of projecting evanescent waves into the near field of the superlens and only for evanescent waves with transverse magnetic component polarization. Finally, plasmonic materials and metamaterials both exhibit significant loss, which reduces the quality of the image.
Fabricating a perfect lens that captures high resolution images of sub-wavelength features of objects has long been desired by lens makers and microscope manufacturers. Lenses that are capable of projecting evanescent waves scattered from of object into the far field are desired.